Plasma etchers are frequently used in semiconductor processing when a relatively straight vertical edge is needed. For instance, when etching the polysilicon gate of a MOS transistor, undercutting the polysilicon can adversely affect the operation of the transistor. Undercutting is frequently encountered when etching is performed using a liquid etching method. Plasma etching, which uses ions accelerated by an electric field, tends to etch only horizontal exposed surfaces and therefore avoids undercutting.
An important aspect of all etching processes is stopping the etching process after the layer being etched has been removed but before the next layer down is destroyed. This is often called "endpoint" detection--for detecting the completion of etching of a particular layer. While the chemicals used in an etching process are selected for their ability to etch a particular type of material, such as polysilicon, the etching chemicals will dissolve the other materials on a semiconductor wafer if those materials are exposed to the etching chemicals for a sufficient period of time. For example, most etching chemicals used to etch polysilicon will also etch silicon oxide. If the silicon oxide used for forming a MOS transistor is completely etched away, the transistor will be destroyed and the wafer being processed will be useless.
In the past, the gate oxides on typical MOS and CMOS circuits have been sufficiently thick (e.g., 500 to 1000 angstroms) that it was relatively easy to stop the polysilicon etching process before the gate oxide overlaying the source and drain regions of transistors was destroyed. However, the device geometries in more advanced circuits have been decreasing in size, resulting in the need for much thinner gate oxides than have been used in the past. At the time of the filing of this document, the assignee of this invention is using gate oxide thicknesses of 200 angstroms on some devices, and is planning to use gate oxides of 175 angstroms or less in the near future.
One consequence of using very thin gate oxides, such as oxides 200 angstroms thick, is that the etching chemical used in a typical plasma etcher when etching polysilicon will attack and destroy the underlying gate oxide much more quickly than when thicker gate oxides were used. In fact, it was found that some wafers were being destroyed by the inadvertent etching of gate oxide, despite the use of an etching system (such as the GCA Waferetch 616 triode etcher) which had state of the art "endpoint" detection equipment. Furthermore, these failures were intermittent. The present invention is the result of the inventors' investigation into the causes of these failures.
Referring to FIG. 1, there is shown a triode etcher system 100. The system 100 has an etching chamber 102 with upper and lower cathodes 104 and 106, respectively, and a screen anode 108. The screen anode 108 is located between the two cathodes and is grounded. A semiconductor wafer 110 is placed in the chamber on the lower cathode 106. In this example the wafer 110 has a silicon substrate 112 which supports an oxide layer 114 and a polysilicon layer 116. The polysilicon layer 116 has been masked with resist 118 to define the areas of polysilicon that are to remain at the end of the etching process. The interior of the etching chamber is filled with a gaseous etching plasma 120.
The system has a 13.56 megahertz RF power supply 130 which has a characteristic impedance of 50 ohms. The chamber 102, however, has a characteristic impedance of around 1000 ohms at this frequency. Without the use of a compensating circuit, this impedance mismatch would cause most of the power output by the power supply to be reflected off the load (i.e., the chamber) and back to the source, which could damage the power supply 130. To overcome this problem, most or all etching systems using a compensating circuit 132, sometimes called an impedance transformation circuit, which matches the amplifier to the plasma. In a triode etcher such as the one shown in FIG. 1 this circuit includes an inductor coil L1 and three tunable capacitors C1, C2 and C3. A controller 134 automatically monitors the reflected power and adjusts the three capacitors until the reflected power is less than a specified threshold value, and also splits the power between the upper and lower electrodes. The exact manner in which the capacitors are adjusted in prior art devices such as the GCA Waferetch 616 triode etcher is not relevant to the present invention. As will be described in more detail below, the aspect of capacitor adjustment which is relevant to the present invention is a new method of adjusting the final adjusted capacitor values so as to improve plasma etching endpoint detection.
In general, the plasma 120 etches the top layer of the wafer 110 only when the power supply 130 is activated and when the power reflected by the plasma chamber is relatively low. Activating the power supply 130 "strikes" the plasma, and activates the etching process. While etching any particular layer, light is generated at frequencies corresponding to the chemical makeup of the layer being etched. That is, the layer being etched chemically combines with the plasma, creating predictable chemical compounds, and the light frequencies present in the plasma correspond to these chemical compounds.
In many plasma etching systems the endpoint of the etching process is detected using a light or optical sensor 140. Typically, the optical sensor 140 is set up, using narrow bandpass filters, to monitor the intensity of light at a frequency associated with the layer being etched. When the measured intensity falls below a specified threshold, indicating that etching is complete, the sensor 140 generates an endpoint signal that is transmitted over line 142 to the controller 134, which turns off the power supply 130 and thereby stops the etching process.
FIG. 2 shows a plasma etcher using a diode etcher system 150 which is similar the triode system shown in FIG. 1. The primary difference is that the diode etcher has only one cathode 152, and thus has a simpler impedance transformation circuit 154 with only two tunable capacitors Cl and C2. The present invention works equally well with diode and triode etcher systems.
The problem solved by the present invention is as follows. In particular, on occasion, while etching the polysilicon gate layer of MOS devices, the gate oxide layer below the polysilicon is etched and destroyed before an endpoint signal is generated by the light sensor 140. This problem, which occurs with both diode and triode plasma etchers, completely destroys the wafer being etched. Therefore, even though this problem does not occur very often, it is still costly.
The inventors of the present invention have found that the problem is not due to any failure in the light sensors 140, but rather with the manner in which the impedance transformation circuit 132 or 154 is adjusted.